3.10 POLYMERIZATION Polymerization is the formation of extremely long molecules from small molecules called monomers. The plastics and rubber are examples of the most common polymers which are commonly used in both everyday life and in medical applications. The exact properties of polymers depends on a variety of chemical details and this unit will give you a brief introduction into some of the common polymers and how their properties depend on their specific chemical details. Polymers can be formed by a variety of reaction pathways. In this unit we will look at those formed by the process of free radical polymerization. Suppose we have a container full of ethene (ethylene) monomers and we unpair the pi electrons by a suitable method, either by appropriate radiation, or a free radical initiator such as a peroxide molecule (the most common way of producing them industrially). By one means or another, we produce free radical ethylene molecules which can pair up with each other to form a long chain: Our example just shows one molecule of monomer as a reactant, although in fact there are large numbers of them that will react with each other to form the polymer: The one sided arrows indicate that one electron goes to each C atom. This is in contrast to the previous reaction pathways where both electrons went to the same atom and we showed a double sided arrow. The ~ indicates that the chain can extend for many thousands (or even millions) of C atoms. What sort of functional group have we produced in the product? It’s an alkane. However, it is named on the basis of the fact that it is formed from polymerization of ethylene: polyethylene. 31 We can diagrammatically represent this polymerization reaction with a monomer, A, being polymerized into a long chain of A: Many A monomers ____> ~A-A-A-A-A-A-A~ Polyethylene The long chains of A can line up to each other somewhat like adjacent strands of spaghetti in a very fairly compact mass and be extruded in thin sheets to make plastic. This form of polyethylene is called high density polyethylene, abbreviated HDPE and is labeled on the plastic recycling system as #2. It is used for laundry detergent jugs, plastic milk bottles, fuel tanks, Tupperware and wood plastic composites which are resistant to rot. Branches in this long polyethylene chain can be introduced so the chains look more like: In this case the chains of somewhat branched polyethylene cannot snuggle as closely together and we have low density polyethylene, abbreviated LDPE, # 4 on the plastics recycling numbering system. 32 The attractions between the strands are not as strong since the chains are held somewhat apart by the branches, but the plastic is less opaque than HDPE. It is used in polyethylene bags, 6-pack plastic rings, and the inside coating of soda cans where it protects the aluminum can from corrosion by the acid of the soda. A third form of polyethylene has extensive cross-linking between the long chains of polyethylene and is called PEX (polyethylene crosslinked) Schematically it looks like PEX is much stronger than HDPE or LDPE due to the cross-links and PEX is seeing increased use in water pipe systems. It is very strong (due to the cross- linking) but still has enough flexibility to expand if the water freezes inside it, so that PEX does not typically crack if the PEX pipes are frozen during cold weather. This is a distinct advantage over steel, copper, and PVC pipes which often crack if water freezes inside them. In medical applications PEX is being used as a low-friction liner between the acetabulum and femoral head in total hip replacements. The PEX provides a very durable, low friction surface between the metal head (typically made of titanium or high grade stainless steel) and the acetabulum.(Can’t just squirt some oil in like the Tin Woodman!) 33 Polypropylene λ ______________> Propene Propylene(common name) = Polypropylene Polypropylene is commonly used to make plastic rope This rope is commonly used in marine applications because it floats. (Can you suggest why?). It is also used for medical instruments that will be autoclaved at temperatures at 120oC. It is 34 also seeing increased used in plastic containers such as yogurt containers. Polypropylene can withstand this temperature where polyethylene cannot. It is also used for making medical suture thread, although nylon is more common. It does have the recycle designated number of 5, although polypropylene is not currently recycled very much. Thought question: What effect will adding a small amount of 1- propene to a polymerization mixture of ethene have? What sort of polyethylene will result? Teflon 1,1,2,2-tetrafluorethene polytetrafluoroethylene (PTFE orTeflon) Besides use in coating non-stick cookware, PTFE membrane is the basis for Gore- tex outdoor clothing. The Gore-tex is woven in a microscopic level fabric that allows water vapor molecule to pass out, but is too small to allow water droplets to pass in, thus providing a raincoat that breathes. 35 Gore-tex is also used in a variety of medical applications such as replacing portions of blood vessels that are damaged or clogged with atherosclerotic plaque with a piece of Gore-tex tube called a graft. (Normally surgeons prefer to take a piece of similar sized vein from the same patient but this is not always possible if the replacement is on a very large blood vessel such as the aorta or if multiple repairs are needed.) Polyacrylonitrile ____> acrylonitrile polyacrylonitrile (PAN) (Orlon) Polyacrylonitrile is being used to make rubber gloves that substitute for latex gloves. 36 Polystyrene _______> Free radical initiator polystyrene If air or other gases are blown into the polymer when it is synthesized we end up with Styrofoam, a very common insulating material used in packaging and in coffee cups, to name just a few uses. Draw the structure of the polymerization product when the monomers shown below are polymerized: Many: ____________________> ____________________> 37 λ ____________> The common name of the last monomer above is vinyl pyrrolidone. The polyvinylpyrrolidone (PVP) polymer has many uses. Medical uses include combination with iodine to make Betadine, which releases iodine slowly into wounds and kills bacteria. It is also mixed with other polymers to make copolymers. Polyvinylidene chloride (Saran) Many Vinylidene chloride polyvinylidene chloride (1,1 dichloroethene) marketed as SaranR Sheets of polyvinylidene(Saran) are marketed for wrapping food and keeping it fresh. The Saran polymer is impermeable to oxygen in the air and hence limits oxygen from getting into the food reducing the rate at which it gets stale; also volatile aroma molecules typically don’t get out of the wrapped food and this helps keep it tasting fresh. On the downside, chlorinated polymer molecules break down only very slowly and hence make a significant contribution to the buildup of non-biodegradable trash. 38 Polyvinylchloride (PVC) Many vinyl chloride polyvinylchloride(PVC) Plasticizers Pure polyvinyl chloride (and some other plastics) can be quite brittle and non- flexible. In order to make it more flexible, small plasticizer molecules are added which slip in between long strands of polymer, lubricating them and making the whole mass more flexible. The most common plasticizer for PVC has been the molecule di (2-ethyl hexyl) phthalate, affectionately abbreviated DEHP. 39 DEHP Identify the functional groups found in DEHP. Can you identify the ethyl and hexyl goups in this structure? DEHP makes PVC more flexible. Very small amounts added to PVC give it slight flexibility while still retaining a pretty rigid form. Addition of larger amounts increases flexibility still further. Concern has been raised by a variety of consumer advocate groups that very small amounts of the DEHP plasticizer may act as an endocrine disruptor and may alter biological hormone levels. This is of particular concern for pregnant women. Concerns have also been raised about the plastics used for food containers especially during heating in microwave ovens. Heating food with plastic containers containing plasticizers can increase the amount of plasticizers that leach out into the food. The amount is fairly small and plastic manufacturers generally claim it is too small to be of concern. Some consumer advocates argue that even though the amounts are small some of these plasticizers can act as carcinogens or endocrine disruptors which can act as hormones and change biological activities in potentially very disruptive ways. There is particular concern about the effect of these chemicals on the fetus. 40 The addition of large amounts of DEHP produce a very flexible plastic which is used in a large variety of plastic applications including medical equipment such as catheters and IV bags. Very small amounts of DEHP can dissolve into the contents of the catheter, IV bags and other medical devices and be transported directly into the bloodstream. Even though the amounts are very small, it is directly entering the bloodstream! This possibility has led to a lot of controversy and the reformulation of many medical plastic items to remove DEHP. Polyethylene, and polyurethane have been substituted as well as the use of plasticizers other than DEHP. [Smell of plasticizer in new cars] Polybutadiene(skip) 41 1.3 butadiene Note that 1.3 butadiene has two double bonds rather than just one. This creates more options during the polymerization process. When the first double bond becomes a free radical, the free radical on the # 1 C atom can pair with another free radical on a butadiene. This leaves an unpaired electron on C # 2 which can react with another butadiene as shown below: butadiene This type of reaction is called a 1,2 addition because the first and second C’s on the molecule are the ones which react.